Automobile engines are comprised of a vaporized fuel processing system in which the vaporized fuel gas (hereinafter referred to as vapor) that is generated in the fuel tank is adsorbed by active carbon in a canister, and under designated operational conditions, by using the negative-pressure of the inlet passage, the fuel particles adsorbed in the canister are desorbed from the active carbon and led to an inlet pipe, which is downstream of a throttle valve, and combusted.
In this case, if a leak hole exists in the middle of a flow path from the fuel tank to the inlet pipe, or if sealing at the joint of the inlet pipe becomes poor, the vapor is discharged to the atmosphere and therefore, leak diagnostic methods have been proposed. U.S. Pat. No. 6,321,727 discloses a leak diagnostic apparatus for a vaporized fuel processing system. However, the leak diagnostic process is not started until the engine is stopped and the leak diagnostic apparatus confirms that the temperature inside the fuel tank is the same or greater than a designated value, relative to the atmosphere temperature.